This invention relates to conductive rubber compositions, and more particularly, to conductive rubber compositions which have improved storage stability, experience minimal changes with time of curability, and cure into conductive rubber experiencing minimal changes with time of volume resistivity. The term xe2x80x9cconductivexe2x80x9d used herein refers to electrical conduction throughout the specification.
Conductive silicone rubber compositions cure into silicone rubber parts having excellent conductivity and are utilized in special applications where heat resistance, flexing resistance and conductivity are required at the same time. A typical conductive silicone rubber composition is disclosed in U.S. Pat. No. 5,173,765 as comprising an organopoly-siloxane having at least two alkenyl groups per molecule, an organohydrogenpolysiloxane having at least two silicon-bonded hydrogen atoms per molecule, a platinum catalyst, and silver powder.
The silver powders used in such conductive silicone rubber are generally divided into three types, a reduced silver powder obtained by reducing an aqueous solution of silver nitrate with a reducing agent such as hydrazine, formaldehyde or ascorbic acid, an electrolytic silver powder obtained by subjecting an aqueous solution of silver nitrate to electrolysis for silver to precipitate on the cathode, and an atomized silver powder obtained by heating silver at 1,000xc2x0 C. or higher and atomizing the molten silver into water or inert gas. With respect to their shape, these powders take the form of granular, flake, dendrite and irregular particles. Of these, flake silver powder is advantageously used because silicone rubber articles formed are highly conductive.
Recently, attempts have been made to blend silver powders which have been surface treated with organosilicon compounds or fluorinated polyether, for improving the storage stability of silicone rubber compositions and reducing the change with time of curability and volume resistivity.
However, when flake silver powder is blended in the conductive silicone rubber composition disclosed in U.S. Pat. No. 5,173,765, a problem arises during shelf storage that silver particles undergo phase separation from the composition. Also, the composition becomes less curable with the passage of time and reaches a point where it is uncurable. Besides, the conductive silicone rubber obtained by curing the composition experiences a considerable change of volume resistivity with time, and is thus unsuitable for everlasting electrical connection between conductors.
The surface treatment of silver powder with silicone compounds (see EP 647682) is effective for improving the affinity of silver powder to silicone rubber compositions, but insufficient to suppress any change of volume resistivity by thermal hysteresis.
An object of the present invention is to provide conductive rubber compositions which have improved storage stability, experience minimal changes with time of curability, and cure into conductive rubber experiencing minimal changes with time of volume resistivity.
It has been found effective to blend a curable polymer with conductive particles containing at least 50% by weight of conductive particles having a specific gravity which differs within xc2x11.5 from the specific gravity of the curable polymer and coated with a metal such as gold, silver or nickel on their outermost layer surface.
Among prior art liquid materials filled with metal powder for developing conductivity, those filled with silver particles are most commonly used. In the known technology, silver particles may have any desired shape, for example, granular, spherical, dendrite, flake and irregular shapes, and mixtures thereof. It is known that silver particles of dendrite or flake shape are chosen for developing satisfactory conductivity, and better conductivity is developed using silver particles of flake shape. It is also known to treat such metal particles with an organosiloxane or fluorine compound for preventing the particles from separating from the matrix resin.
Even when these techniques were used, it was difficult to avoid separation of conductive particles from the matrix resin during a long period of storage. Partial changes of conductivity during storage were observed. It was also ascertained that conductive elastomers after curing changed their conductivity by thermal shocks.
With these borne in mind, the inventor studied the nature of conductive particles. And the inventor has succeeded in minimizing the separation of conductive particles and the change of volume resistivity by thermal shocks, by controlling the specific gravity of conductive particles so that the difference between the specific gravity of conductive particles and the specific gravity of the matrix resin falls within xc2x11.5.
According the invention, there is provided a liquid conductive composition comprising (a) a curable polymer and (b) conductive particles in which those conductive particles coated with a metal on their outermost layer surface and having a specific gravity which differs within xc2x11.5 from the specific gravity of the curable polymer (a) account for at least 50% by weight of the entire conductive particles (b).